This concerns a transistor circuit. In particular, it concerns a transistor circuit which provides a constant-current source circuit which can operate with a low power source voltage threshold and is minimally dependent on its electric power source, and a constant-voltage circuit which makes use of these properties of the constant-current circuit.
In recent years, with the miniaturization of portable audio equipment, cameras, etc., the demand has increased for constant-current and constant-voltage circuits which are unaffected, even at low voltages, by variations in power source voltage, temperature, etc.
Various type of constant-current and constant-voltage circuits have been proposed before. The present invention aims to meet the demand referred to above by improving on such circuits. An explanation is given first of the prior art.
FIG. 1(A) is a constant-current source which makes use of a current mirror circuit. The two emitters, and the two bases, of the two NPN type transistors 1 and 2 are respectively connected in common. The emitters are connected to the first power supply terminal G. The collector and base of the transistor 1 are connected together, and via the resistor 3 are connected to the second power supply terminal V.sub.CC. Since the voltage V.sub.BE between the base and emitter of each of the transistors 1 and 2 is the same, the collector currents of the two transistors will be equal if their structural dimensions are the same; and consequently the output current I.sub.out can be expressed as follows ##EQU1## where R is the resistance of the resistor 3 and V.sub.CC the voltage at terminal V.sub.CC.
Since however the output current I.sub.out depends on the voltage V.sub.CC of the power supply, the properties of the circuit are less than satisfactory, as is shown by FIG. 1(B).
FIG. 2 shows examples of conventional circuits which have been improved by reducing the effect of the power supply on the current values. If the structural dimensions of the transistors 21, 22 and 23, and 26 and 27 in the layout of the circuits shown in FIGS. 2(A) and (B) are all made equal, a voltage equal to the voltage V.sub.BE between the base and emitter of the transistors will be produced at the resistors 24 and 28 shown in the circuits. Consequently, if the base-common-current amplification factor .alpha. of each of the transistors 22 and 27 is taken as being 1, the output current I.sub.out is expressed in each case by EQU I.sub.out =V.sub.BE /R (2)
where R is the resistance of the resistors 24 and 28.
As formula (2) shows, in a current source circuit of this type the output current I.sub.out does not depend on the voltage V.sub.CC of the power supply. However, such circuits will not operate unless the base potential of the respective transistors 22 or 27 is at least twice the voltage V.sub.BE between base and emitter. Their operating properties are shown in FIG. 2(c), and there is no output current I.sub.out until V.sub.CC exceeds 1.4 V.
FIG. 2(D) is an example of a current mode logic (CML) circuit using the circuit illustrated in FIG. 2(A) as its current source. The common emitters of the two transistors 30-1 and 30-2 are connected to the collector of the transistor 22; the collector of each of the transistors 30-1 and 30-2 is connected, via the resistors 30-3 and 30-4, to the power supply terminal V.sub.CC. One of the transistors 30-1 and 30-2 turns ON as a result of the relationship between the electric potentials of the inputs applied to the respective bases of the two transistors; and the output is obtained via the resistor 30-3 or the resistor 30-4. This circuit will not operate unless at least the electric potential of the collector of the transistor 21 is more than 2V.sub.BE, i.e. at least approximately 1.4 V; and this means that the electric potential of the collector of the transistor 22 must also be at least 1.4 V. Further, for the logic circuit to respond to the inputs applied to the bases of the transistors 30-1 and 30-2, a voltage applied to these bases must be at least V.sub.BE (i.e. 0.7 V) added to 1.4 V.
Thus a voltage of at least 2.1 V must be applied to the collectors of transistors 30-1 and 30-2. Therefore the power supply voltage must be at least 2.1 V.
FIG. 3 shows a conventional circuit which has been improved to reduce the threshold voltage of a constant-current source circuit. The transistor 31 is biased by the series circuit of resistors 33 and 34 connected between the base of the transistor 32 and the power supply terminal G. We can assume that the relation between the resistances R.sub.33 and R.sub.34 of the resistors 33 and 34 is set at EQU R.sub.34 =k.multidot.R.sub.33 ( 3).
Then, if the voltage drop across R.sub.33 is greater than the base-emitter voltage V.sub.BE of the transistor, i.e. at least approximately 0.7 V, the transistor 31 will be in a conducting state. Since the base-emitter voltage V.sub.BE of the transistor 31 is kept constant at approximately 0.7 V even though the power supply voltage V.sub.CC may be larger the base potential of the transistor 32 is also kept constant at (1+k).multidot.V.sub.BE.
Consequently, since the voltage drop occurring across the resistor 35 connected between the emitter of the transistor 32 and the power supply terminal G is k.multidot.V.sub.BE, the collector current of the transistor 32, or in other words the output current I.sub.out, can be expressed as follows EQU I.sub.out =k.multidot.V.sub.BE /R.sub.35 ( 4)
where R.sub.35 is the resistance of the resistor 35. But this type of circuit will not operate unless the power supply voltage V.sub.CC is more than (1+k)V.sub.BE, as shown in FIG. 3(B).